Rumblings below after those horrid starters (7)
I believe the answer is:
thunder
'rumblings' is the definition.
(I have seen 'Meteorological rumblings' mean 'thunder' so perhaps 'rumblings' could also mean 'thunder')
'below after those horrid starters' is the wordplay.
'below' becomes 'under' (synonyms).
'after' means one lot of letters go next to another.
'starters' says to take the initial letters.
The initial letters of 'those horrid' is 'th'.
'under' put after 'th' is 'THUNDER'.
(Other definitions for thunder that I've seen before include "Booming noise from the heavens" , "It accompanies lightning" , "Meteorological rumblings" , "What comes with lightning" , "Booming noise after lightning" .)